diff mbox series

[RFC,v3,03/27] qcow2: Process QCOW2_CLUSTER_ZERO_ALLOC clusters in handle_copied()

Message ID e327f4c1ed2f9626ce018c1fd2b9db437721b30c.1577014346.git.berto@igalia.com
State New
Headers show
Series Add subcluster allocation to qcow2 | expand

Commit Message

Alberto Garcia Dec. 22, 2019, 11:36 a.m. UTC 
When writing to a qcow2 file there are two functions that take a
virtual offset and return a host offset, possibly allocating new
clusters if necessary:

   - handle_copied() looks for normal data clusters that are already
     allocated and have a reference count of 1. In those clusters we
     can simply write the data and there is no need to perform any
     copy-on-write.

   - handle_alloc() looks for clusters that do need copy-on-write,
     either because they haven't been allocated yet, because their
     reference count is != 1 or because they are ZERO_ALLOC clusters.

The ZERO_ALLOC case is a bit special because those are clusters that
are already allocated and they could perfectly be dealt with in
handle_copied() (as long as copy-on-write is performed when required).

In fact, there is extra code specifically for them in handle_alloc()
that tries to reuse the existing allocation if possible and frees them
otherwise.

This patch changes the handling of ZERO_ALLOC clusters so the
semantics of these two functions are now like this:

   - handle_copied() looks for clusters that are already allocated and
     which we can overwrite (NORMAL and ZERO_ALLOC clusters with a
     reference count of 1).

   - handle_alloc() looks for clusters for which we need a new
     allocation (all other cases).

One importante difference after this change is that clusters found in
handle_copied() may now require copy-on-write, but this will be anyway
necessary once we add support for subclusters.

Signed-off-by: Alberto Garcia <berto@igalia.com>
---
 block/qcow2-cluster.c | 226 +++++++++++++++++++++++-------------------
 1 file changed, 126 insertions(+), 100 deletions(-)

Comments

Eric Blake Feb. 20, 2020, 2:53 p.m. UTC | #1 
On 12/22/19 5:36 AM, Alberto Garcia wrote:
> When writing to a qcow2 file there are two functions that take a
> virtual offset and return a host offset, possibly allocating new
> clusters if necessary:
> 
>     - handle_copied() looks for normal data clusters that are already
>       allocated and have a reference count of 1. In those clusters we
>       can simply write the data and there is no need to perform any
>       copy-on-write.
> 
>     - handle_alloc() looks for clusters that do need copy-on-write,
>       either because they haven't been allocated yet, because their
>       reference count is != 1 or because they are ZERO_ALLOC clusters.
> 
> The ZERO_ALLOC case is a bit special because those are clusters that
> are already allocated and they could perfectly be dealt with in
> handle_copied() (as long as copy-on-write is performed when required).
> 
> In fact, there is extra code specifically for them in handle_alloc()
> that tries to reuse the existing allocation if possible and frees them
> otherwise.
> 
> This patch changes the handling of ZERO_ALLOC clusters so the
> semantics of these two functions are now like this:
> 
>     - handle_copied() looks for clusters that are already allocated and
>       which we can overwrite (NORMAL and ZERO_ALLOC clusters with a
>       reference count of 1).
> 
>     - handle_alloc() looks for clusters for which we need a new
>       allocation (all other cases).
> 
> One importante difference after this change is that clusters found in

important

> handle_copied() may now require copy-on-write, but this will be anyway
> necessary once we add support for subclusters.

necessary anyway

> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
>   block/qcow2-cluster.c | 226 +++++++++++++++++++++++-------------------
>   1 file changed, 126 insertions(+), 100 deletions(-)

A bit of an increase in code size, but I think it does reduce the 
overall complexity to treat ZERO_ALLOC like normal.  The patch is big, 
but I don't see any sane way to split it.  Overall, I like it.

> 
> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> index e078bddcc2..9387f15866 100644
> --- a/block/qcow2-cluster.c
> +++ b/block/qcow2-cluster.c

> @@ -1069,18 +1112,20 @@ static void calculate_l2_meta(BlockDriverState *bs,
>       QLIST_INSERT_HEAD(&s->cluster_allocs, *m, next_in_flight);
>   }
>   
> -/* Returns true if writing to a cluster requires COW */
> -static bool cluster_needs_cow(BlockDriverState *bs, uint64_t l2_entry)
> +/* Returns true if writing to the cluster pointed to by @l2_entry
> + * requires a new allocation (that is, if the cluster is unallocated
> + * or has refcount > 1 and therefore cannot be written in-place). */

syntax check wants you to wing this comment, now.

> +static bool cluster_needs_new_alloc(BlockDriverState *bs, uint64_t l2_entry)

The rename makes sense.

> @@ -1337,9 +1400,10 @@ static int do_alloc_cluster_offset(BlockDriverState *bs, uint64_t guest_offset,
>   }
>   
>   /*
> - * Allocates new clusters for an area that either is yet unallocated or needs a
> - * copy on write. If *host_offset is not INV_OFFSET, clusters are only
> - * allocated if the new allocation can match the specified host offset.
> + * Allocates new clusters for an area that either is yet unallocated or
> + * cannot be overwritten in-place. If *host_offset is not INV_OFFSET,

s/either is yet/is either still/

> + * clusters are only allocated if the new allocation can match the specified
> + * host offset.
>    *
>    * Note that guest_offset may not be cluster aligned. In this case, the
>    * returned *host_offset points to exact byte referenced by guest_offset and

Findings are minor and you can fix them up when dropping RFC.

Reviewed-by: Eric Blake <eblake@redhat.com>
Max Reitz Feb. 20, 2020, 3 p.m. UTC | #2 
On 22.12.19 12:36, Alberto Garcia wrote:
> When writing to a qcow2 file there are two functions that take a
> virtual offset and return a host offset, possibly allocating new
> clusters if necessary:
> 
>    - handle_copied() looks for normal data clusters that are already
>      allocated and have a reference count of 1. In those clusters we
>      can simply write the data and there is no need to perform any
>      copy-on-write.
> 
>    - handle_alloc() looks for clusters that do need copy-on-write,
>      either because they haven't been allocated yet, because their
>      reference count is != 1 or because they are ZERO_ALLOC clusters.
> 
> The ZERO_ALLOC case is a bit special because those are clusters that
> are already allocated and they could perfectly be dealt with in
> handle_copied() (as long as copy-on-write is performed when required).
> 
> In fact, there is extra code specifically for them in handle_alloc()
> that tries to reuse the existing allocation if possible and frees them
> otherwise.
> 
> This patch changes the handling of ZERO_ALLOC clusters so the
> semantics of these two functions are now like this:
> 
>    - handle_copied() looks for clusters that are already allocated and
>      which we can overwrite (NORMAL and ZERO_ALLOC clusters with a
>      reference count of 1).
> 
>    - handle_alloc() looks for clusters for which we need a new
>      allocation (all other cases).
> 
> One importante difference after this change is that clusters found in
> handle_copied() may now require copy-on-write, but this will be anyway
> necessary once we add support for subclusters.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
>  block/qcow2-cluster.c | 226 +++++++++++++++++++++++-------------------
>  1 file changed, 126 insertions(+), 100 deletions(-)

Reviewed-by: Max Reitz <mreitz@redhat.com>
Max Reitz Feb. 20, 2020, 3:19 p.m. UTC | #3 
I have no idea how I managed to forgot some style comments I wrote
during the review, but, anyway:

On 22.12.19 12:36, Alberto Garcia wrote:
> When writing to a qcow2 file there are two functions that take a
> virtual offset and return a host offset, possibly allocating new
> clusters if necessary:
> 
>    - handle_copied() looks for normal data clusters that are already
>      allocated and have a reference count of 1. In those clusters we
>      can simply write the data and there is no need to perform any
>      copy-on-write.
> 
>    - handle_alloc() looks for clusters that do need copy-on-write,
>      either because they haven't been allocated yet, because their
>      reference count is != 1 or because they are ZERO_ALLOC clusters.
> 
> The ZERO_ALLOC case is a bit special because those are clusters that
> are already allocated and they could perfectly be dealt with in
> handle_copied() (as long as copy-on-write is performed when required).
> 
> In fact, there is extra code specifically for them in handle_alloc()
> that tries to reuse the existing allocation if possible and frees them
> otherwise.
> 
> This patch changes the handling of ZERO_ALLOC clusters so the
> semantics of these two functions are now like this:
> 
>    - handle_copied() looks for clusters that are already allocated and
>      which we can overwrite (NORMAL and ZERO_ALLOC clusters with a
>      reference count of 1).
> 
>    - handle_alloc() looks for clusters for which we need a new
>      allocation (all other cases).
> 
> One importante difference after this change is that clusters found in

s/importante/important/

> handle_copied() may now require copy-on-write, but this will be anyway
> necessary once we add support for subclusters.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
>  block/qcow2-cluster.c | 226 +++++++++++++++++++++++-------------------
>  1 file changed, 126 insertions(+), 100 deletions(-)
> 
> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> index e078bddcc2..9387f15866 100644
> --- a/block/qcow2-cluster.c
> +++ b/block/qcow2-cluster.c

[...]

> @@ -1035,15 +1040,53 @@ void qcow2_alloc_cluster_abort(BlockDriverState *bs, QCowL2Meta *m)
>  static void calculate_l2_meta(BlockDriverState *bs,
>                                uint64_t host_cluster_offset,
>                                uint64_t guest_offset, unsigned bytes,
> -                              QCowL2Meta **m, bool keep_old)
> +                              uint64_t *l2_slice, QCowL2Meta **m, bool keep_old)
>  {

[...]

> +    /* Return if there's no COW (all clusters are normal and we keep them) */
> +    if (keep_old) {
> +        int i;
> +        for (i = 0; i < nb_clusters; i++) {
> +            l2_entry = be64_to_cpu(l2_slice[l2_index + i]);
> +            if (qcow2_get_cluster_type(bs, l2_entry) != QCOW2_CLUSTER_NORMAL) {
> +                break;
> +            }
> +        }
> +        if (i == nb_clusters) {
> +            return;
> +        }
> +    }
> +
> +    /* Get the L2 entry from the first cluster */

s/from/of/

(Otherwise it sounds a bit like this is the same entry for all clusters)

> +    l2_entry = be64_to_cpu(l2_slice[l2_index]);
> +    type = qcow2_get_cluster_type(bs, l2_entry);
> +
> +    if (type == QCOW2_CLUSTER_NORMAL && keep_old) {
> +        cow_start_from = cow_start_to;
> +    } else {
> +        cow_start_from = 0;
> +    }
> +
> +    /* Get the L2 entry from the last cluster */

s/from/of/

> +    l2_entry = be64_to_cpu(l2_slice[l2_index + nb_clusters - 1]);
> +    type = qcow2_get_cluster_type(bs, l2_entry);
> +
> +    if (type == QCOW2_CLUSTER_NORMAL && keep_old) {
> +        cow_end_to = cow_end_from;
> +    } else {
> +        cow_end_to = ROUND_UP(cow_end_from, s->cluster_size);
> +    }
>  
>      *m = g_malloc0(sizeof(**m));
>      **m = (QCowL2Meta) {
> @@ -1069,18 +1112,20 @@ static void calculate_l2_meta(BlockDriverState *bs,
>      QLIST_INSERT_HEAD(&s->cluster_allocs, *m, next_in_flight);
>  }
>  
> -/* Returns true if writing to a cluster requires COW */
> -static bool cluster_needs_cow(BlockDriverState *bs, uint64_t l2_entry)
> +/* Returns true if writing to the cluster pointed to by @l2_entry
> + * requires a new allocation (that is, if the cluster is unallocated
> + * or has refcount > 1 and therefore cannot be written in-place). */

Not sure why Patchew hasn’t complained, but the current coding style
requires /* and */ to be on separate lines for multi-line comments.

Max
diff mbox series

Patch

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index e078bddcc2..9387f15866 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1021,13 +1021,18 @@  void qcow2_alloc_cluster_abort(BlockDriverState *bs, QCowL2Meta *m)
 
 /*
  * For a given write request, create a new QCowL2Meta structure, add
- * it to @m and the BDRVQcow2State.cluster_allocs list.
+ * it to @m and the BDRVQcow2State.cluster_allocs list. If the write
+ * request does not need copy-on-write or changes to the L2 metadata
+ * then this function does nothing.
  *
  * @host_cluster_offset points to the beginning of the first cluster.
  *
  * @guest_offset and @bytes indicate the offset and length of the
  * request.
  *
+ * @l2_slice contains the L2 entries of all clusters involved in this
+ * write request.
+ *
  * If @keep_old is true it means that the clusters were already
  * allocated and will be overwritten. If false then the clusters are
  * new and we have to decrease the reference count of the old ones.
@@ -1035,15 +1040,53 @@  void qcow2_alloc_cluster_abort(BlockDriverState *bs, QCowL2Meta *m)
 static void calculate_l2_meta(BlockDriverState *bs,
                               uint64_t host_cluster_offset,
                               uint64_t guest_offset, unsigned bytes,
-                              QCowL2Meta **m, bool keep_old)
+                              uint64_t *l2_slice, QCowL2Meta **m, bool keep_old)
 {
     BDRVQcow2State *s = bs->opaque;
-    unsigned cow_start_from = 0;
+    int l2_index = offset_to_l2_slice_index(s, guest_offset);
+    uint64_t l2_entry;
+    unsigned cow_start_from, cow_end_to;
     unsigned cow_start_to = offset_into_cluster(s, guest_offset);
     unsigned cow_end_from = cow_start_to + bytes;
-    unsigned cow_end_to = ROUND_UP(cow_end_from, s->cluster_size);
     unsigned nb_clusters = size_to_clusters(s, cow_end_from);
     QCowL2Meta *old_m = *m;
+    QCow2ClusterType type;
+
+    assert(nb_clusters <= s->l2_slice_size - l2_index);
+
+    /* Return if there's no COW (all clusters are normal and we keep them) */
+    if (keep_old) {
+        int i;
+        for (i = 0; i < nb_clusters; i++) {
+            l2_entry = be64_to_cpu(l2_slice[l2_index + i]);
+            if (qcow2_get_cluster_type(bs, l2_entry) != QCOW2_CLUSTER_NORMAL) {
+                break;
+            }
+        }
+        if (i == nb_clusters) {
+            return;
+        }
+    }
+
+    /* Get the L2 entry from the first cluster */
+    l2_entry = be64_to_cpu(l2_slice[l2_index]);
+    type = qcow2_get_cluster_type(bs, l2_entry);
+
+    if (type == QCOW2_CLUSTER_NORMAL && keep_old) {
+        cow_start_from = cow_start_to;
+    } else {
+        cow_start_from = 0;
+    }
+
+    /* Get the L2 entry from the last cluster */
+    l2_entry = be64_to_cpu(l2_slice[l2_index + nb_clusters - 1]);
+    type = qcow2_get_cluster_type(bs, l2_entry);
+
+    if (type == QCOW2_CLUSTER_NORMAL && keep_old) {
+        cow_end_to = cow_end_from;
+    } else {
+        cow_end_to = ROUND_UP(cow_end_from, s->cluster_size);
+    }
 
     *m = g_malloc0(sizeof(**m));
     **m = (QCowL2Meta) {
@@ -1069,18 +1112,20 @@  static void calculate_l2_meta(BlockDriverState *bs,
     QLIST_INSERT_HEAD(&s->cluster_allocs, *m, next_in_flight);
 }
 
-/* Returns true if writing to a cluster requires COW */
-static bool cluster_needs_cow(BlockDriverState *bs, uint64_t l2_entry)
+/* Returns true if writing to the cluster pointed to by @l2_entry
+ * requires a new allocation (that is, if the cluster is unallocated
+ * or has refcount > 1 and therefore cannot be written in-place). */
+static bool cluster_needs_new_alloc(BlockDriverState *bs, uint64_t l2_entry)
 {
     switch (qcow2_get_cluster_type(bs, l2_entry)) {
     case QCOW2_CLUSTER_NORMAL:
+    case QCOW2_CLUSTER_ZERO_ALLOC:
         if (l2_entry & QCOW_OFLAG_COPIED) {
             return false;
         }
     case QCOW2_CLUSTER_UNALLOCATED:
     case QCOW2_CLUSTER_COMPRESSED:
     case QCOW2_CLUSTER_ZERO_PLAIN:
-    case QCOW2_CLUSTER_ZERO_ALLOC:
         return true;
     default:
         abort();
@@ -1088,20 +1133,36 @@  static bool cluster_needs_cow(BlockDriverState *bs, uint64_t l2_entry)
 }
 
 /*
- * Returns the number of contiguous clusters that can be used for an allocating
- * write, but require COW to be performed (this includes yet unallocated space,
- * which must copy from the backing file)
+ * Returns the number of contiguous clusters that can be written to
+ * using one single write request, starting from @l2_index.
+ * At most @nb_clusters are checked.
+ *
+ * If @new_alloc is true this counts clusters that are either
+ * unallocated, or allocated but with refcount > 1 (so they need to be
+ * newly allocated and COWed).
+ *
+ * If @new_alloc is false this counts clusters that are already
+ * allocated and can be overwritten in-place (this includes clusters
+ * of type QCOW2_CLUSTER_ZERO_ALLOC).
  */
-static int count_cow_clusters(BlockDriverState *bs, int nb_clusters,
-    uint64_t *l2_slice, int l2_index)
+static int count_single_write_clusters(BlockDriverState *bs, int nb_clusters,
+                                       uint64_t *l2_slice, int l2_index,
+                                       bool new_alloc)
 {
+    BDRVQcow2State *s = bs->opaque;
+    uint64_t l2_entry = be64_to_cpu(l2_slice[l2_index]);
+    uint64_t expected_offset = l2_entry & L2E_OFFSET_MASK;
     int i;
 
     for (i = 0; i < nb_clusters; i++) {
-        uint64_t l2_entry = be64_to_cpu(l2_slice[l2_index + i]);
-        if (!cluster_needs_cow(bs, l2_entry)) {
+        l2_entry = be64_to_cpu(l2_slice[l2_index + i]);
+        if (cluster_needs_new_alloc(bs, l2_entry) != new_alloc) {
             break;
         }
+        if (!new_alloc && expected_offset != (l2_entry & L2E_OFFSET_MASK)) {
+            break;
+        }
+        expected_offset += s->cluster_size;
     }
 
     assert(i <= nb_clusters);
@@ -1172,10 +1233,10 @@  static int handle_dependencies(BlockDriverState *bs, uint64_t guest_offset,
 }
 
 /*
- * Checks how many already allocated clusters that don't require a copy on
- * write there are at the given guest_offset (up to *bytes). If *host_offset is
- * not INV_OFFSET, only physically contiguous clusters beginning at this host
- * offset are counted.
+ * Checks how many already allocated clusters that don't require a new
+ * allocation there are at the given guest_offset (up to *bytes).
+ * If *host_offset is not INV_OFFSET, only physically contiguous clusters
+ * beginning at this host offset are counted.
  *
  * Note that guest_offset may not be cluster aligned. In this case, the
  * returned *host_offset points to exact byte referenced by guest_offset and
@@ -1184,12 +1245,12 @@  static int handle_dependencies(BlockDriverState *bs, uint64_t guest_offset,
  * Returns:
  *   0:     if no allocated clusters are available at the given offset.
  *          *bytes is normally unchanged. It is set to 0 if the cluster
- *          is allocated and doesn't need COW, but doesn't have the right
- *          physical offset.
+ *          is allocated and can be overwritten in-place but doesn't have
+ *          the right physical offset.
  *
- *   1:     if allocated clusters that don't require a COW are available at
- *          the requested offset. *bytes may have decreased and describes
- *          the length of the area that can be written to.
+ *   1:     if allocated clusters that can be overwritten in place are
+ *          available at the requested offset. *bytes may have decreased
+ *          and describes the length of the area that can be written to.
  *
  *  -errno: in error cases
  */
@@ -1219,7 +1280,8 @@  static int handle_copied(BlockDriverState *bs, uint64_t guest_offset,
 
     l2_index = offset_to_l2_slice_index(s, guest_offset);
     nb_clusters = MIN(nb_clusters, s->l2_slice_size - l2_index);
-    assert(nb_clusters <= INT_MAX);
+    /* Limit total byte count to BDRV_REQUEST_MAX_BYTES */
+    nb_clusters = MIN(nb_clusters, BDRV_REQUEST_MAX_BYTES >> s->cluster_bits);
 
     /* Find L2 entry for the first involved cluster */
     ret = get_cluster_table(bs, guest_offset, &l2_slice, &l2_index);
@@ -1229,18 +1291,17 @@  static int handle_copied(BlockDriverState *bs, uint64_t guest_offset,
 
     cluster_offset = be64_to_cpu(l2_slice[l2_index]);
 
-    /* Check how many clusters are already allocated and don't need COW */
-    if (qcow2_get_cluster_type(bs, cluster_offset) == QCOW2_CLUSTER_NORMAL
-        && (cluster_offset & QCOW_OFLAG_COPIED))
-    {
+    if (!cluster_needs_new_alloc(bs, cluster_offset)) {
         /* If a specific host_offset is required, check it */
         bool offset_matches =
             (cluster_offset & L2E_OFFSET_MASK) == *host_offset;
 
         if (offset_into_cluster(s, cluster_offset & L2E_OFFSET_MASK)) {
-            qcow2_signal_corruption(bs, true, -1, -1, "Data cluster offset "
+            qcow2_signal_corruption(bs, true, -1, -1, "%s cluster offset "
                                     "%#llx unaligned (guest offset: %#" PRIx64
-                                    ")", cluster_offset & L2E_OFFSET_MASK,
+                                    ")", cluster_offset & QCOW_OFLAG_ZERO ?
+                                    "Preallocated zero" : "Data",
+                                    cluster_offset & L2E_OFFSET_MASK,
                                     guest_offset);
             ret = -EIO;
             goto out;
@@ -1253,15 +1314,17 @@  static int handle_copied(BlockDriverState *bs, uint64_t guest_offset,
         }
 
         /* We keep all QCOW_OFLAG_COPIED clusters */
-        keep_clusters =
-            count_contiguous_clusters(bs, nb_clusters, s->cluster_size,
-                                      &l2_slice[l2_index],
-                                      QCOW_OFLAG_COPIED | QCOW_OFLAG_ZERO);
+        keep_clusters = count_single_write_clusters(bs, nb_clusters, l2_slice,
+                                                    l2_index, false);
         assert(keep_clusters <= nb_clusters);
 
         *bytes = MIN(*bytes,
                  keep_clusters * s->cluster_size
                  - offset_into_cluster(s, guest_offset));
+        assert(*bytes != 0);
+
+        calculate_l2_meta(bs, cluster_offset & L2E_OFFSET_MASK, guest_offset,
+                          *bytes, l2_slice, m, true);
 
         ret = 1;
     } else {
@@ -1337,9 +1400,10 @@  static int do_alloc_cluster_offset(BlockDriverState *bs, uint64_t guest_offset,
 }
 
 /*
- * Allocates new clusters for an area that either is yet unallocated or needs a
- * copy on write. If *host_offset is not INV_OFFSET, clusters are only
- * allocated if the new allocation can match the specified host offset.
+ * Allocates new clusters for an area that either is yet unallocated or
+ * cannot be overwritten in-place. If *host_offset is not INV_OFFSET,
+ * clusters are only allocated if the new allocation can match the specified
+ * host offset.
  *
  * Note that guest_offset may not be cluster aligned. In this case, the
  * returned *host_offset points to exact byte referenced by guest_offset and
@@ -1362,12 +1426,10 @@  static int handle_alloc(BlockDriverState *bs, uint64_t guest_offset,
     BDRVQcow2State *s = bs->opaque;
     int l2_index;
     uint64_t *l2_slice;
-    uint64_t entry;
     uint64_t nb_clusters;
     int ret;
-    bool keep_old_clusters = false;
 
-    uint64_t alloc_cluster_offset = INV_OFFSET;
+    uint64_t alloc_cluster_offset;
 
     trace_qcow2_handle_alloc(qemu_coroutine_self(), guest_offset, *host_offset,
                              *bytes);
@@ -1382,10 +1444,8 @@  static int handle_alloc(BlockDriverState *bs, uint64_t guest_offset,
 
     l2_index = offset_to_l2_slice_index(s, guest_offset);
     nb_clusters = MIN(nb_clusters, s->l2_slice_size - l2_index);
-    assert(nb_clusters <= INT_MAX);
-
-    /* Limit total allocation byte count to INT_MAX */
-    nb_clusters = MIN(nb_clusters, INT_MAX >> s->cluster_bits);
+    /* Limit total allocation byte count to BDRV_REQUEST_MAX_BYTES */
+    nb_clusters = MIN(nb_clusters, BDRV_REQUEST_MAX_BYTES >> s->cluster_bits);
 
     /* Find L2 entry for the first involved cluster */
     ret = get_cluster_table(bs, guest_offset, &l2_slice, &l2_index);
@@ -1393,67 +1453,32 @@  static int handle_alloc(BlockDriverState *bs, uint64_t guest_offset,
         return ret;
     }
 
-    entry = be64_to_cpu(l2_slice[l2_index]);
-    nb_clusters = count_cow_clusters(bs, nb_clusters, l2_slice, l2_index);
+    nb_clusters = count_single_write_clusters(bs, nb_clusters,
+                                              l2_slice, l2_index, true);
 
     /* This function is only called when there were no non-COW clusters, so if
      * we can't find any unallocated or COW clusters either, something is
      * wrong with our code. */
     assert(nb_clusters > 0);
 
-    if (qcow2_get_cluster_type(bs, entry) == QCOW2_CLUSTER_ZERO_ALLOC &&
-        (entry & QCOW_OFLAG_COPIED) &&
-        (*host_offset == INV_OFFSET ||
-         start_of_cluster(s, *host_offset) == (entry & L2E_OFFSET_MASK)))
-    {
-        int preallocated_nb_clusters;
-
-        if (offset_into_cluster(s, entry & L2E_OFFSET_MASK)) {
-            qcow2_signal_corruption(bs, true, -1, -1, "Preallocated zero "
-                                    "cluster offset %#llx unaligned (guest "
-                                    "offset: %#" PRIx64 ")",
-                                    entry & L2E_OFFSET_MASK, guest_offset);
-            ret = -EIO;
-            goto fail;
-        }
-
-        /* Try to reuse preallocated zero clusters; contiguous normal clusters
-         * would be fine, too, but count_cow_clusters() above has limited
-         * nb_clusters already to a range of COW clusters */
-        preallocated_nb_clusters =
-            count_contiguous_clusters(bs, nb_clusters, s->cluster_size,
-                                      &l2_slice[l2_index], QCOW_OFLAG_COPIED);
-        assert(preallocated_nb_clusters > 0);
-
-        nb_clusters = preallocated_nb_clusters;
-        alloc_cluster_offset = entry & L2E_OFFSET_MASK;
-
-        /* We want to reuse these clusters, so qcow2_alloc_cluster_link_l2()
-         * should not free them. */
-        keep_old_clusters = true;
+    /* Allocate at a given offset in the image file */
+    alloc_cluster_offset = *host_offset == INV_OFFSET ? INV_OFFSET :
+        start_of_cluster(s, *host_offset);
+    ret = do_alloc_cluster_offset(bs, guest_offset, &alloc_cluster_offset,
+                                  &nb_clusters);
+    if (ret < 0) {
+        goto out;
     }
 
-    qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
-
-    if (alloc_cluster_offset == INV_OFFSET) {
-        /* Allocate, if necessary at a given offset in the image file */
-        alloc_cluster_offset = *host_offset == INV_OFFSET ? INV_OFFSET :
-                               start_of_cluster(s, *host_offset);
-        ret = do_alloc_cluster_offset(bs, guest_offset, &alloc_cluster_offset,
-                                      &nb_clusters);
-        if (ret < 0) {
-            goto fail;
-        }
-
-        /* Can't extend contiguous allocation */
-        if (nb_clusters == 0) {
-            *bytes = 0;
-            return 0;
-        }
-
-        assert(alloc_cluster_offset != INV_OFFSET);
+    /* Can't extend contiguous allocation */
+    if (nb_clusters == 0) {
+        *bytes = 0;
+        ret = 0;
+        goto out;
     }
 
+    assert(alloc_cluster_offset != INV_OFFSET);
+
     /*
      * Save info needed for meta data update.
      *
@@ -1476,13 +1501,14 @@  static int handle_alloc(BlockDriverState *bs, uint64_t guest_offset,
     *bytes = MIN(*bytes, nb_bytes - offset_into_cluster(s, guest_offset));
     assert(*bytes != 0);
 
-    calculate_l2_meta(bs, alloc_cluster_offset, guest_offset, *bytes,
-                      m, keep_old_clusters);
+    calculate_l2_meta(bs, alloc_cluster_offset, guest_offset, *bytes, l2_slice,
+                      m, false);
 
-    return 1;
+    ret = 1;
 
-fail:
-    if (*m && (*m)->nb_clusters > 0) {
+out:
+    qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
+    if (ret < 0 && *m && (*m)->nb_clusters > 0) {
         QLIST_REMOVE(*m, next_in_flight);
     }
     return ret;